Seal compositions, methods, and structures for planar solid oxide fuel cells

ABSTRACT

A seal composition includes a first alkaline earth metal oxide, a second alkaline earth metal oxide which is different from the first alkaline earth metal oxide, aluminum oxide, and silica in an amount such that molar percent of silica in the composition is at least five molar percent greater than two times a combined molar percent of the first alkaline earth metal oxide and the second alkaline earth metal oxide. The composition is substantially free of boron oxide and phosphorus oxide. The seal composition forms a glass ceramic seal which includes silica containing glass cores located in a crystalline matrix comprising barium aluminosilicate, and calcium aluminosilicate crystals located in the glass cores.

BACKGROUND

The present invention relates generally to the field of glass ceramicseals and seal compositions, and in particular to those suited for solidoxide fuel cells (SOFCs).

SOFC systems typically employ a seal on a component surface, such as asurface of interconnect elements, to seal the fuel gases from theambient air. However, there is still a need to develop improved seals.Particularly, glass ceramic seals that exhibit little or no degradationfrom contact with the fuel or ambient gases, are of interest.

SUMMARY

One embodiment describes a seal composition includes a first alkalineearth metal oxide, a second alkaline earth metal oxide which isdifferent from the first alkaline earth metal oxide, aluminum oxide, andsilica in an amount such that molar percent of silica in the compositionis at least five molar percent greater than two times a combined molarpercent of the first alkaline earth metal oxide and the second alkalineearth metal oxide. The composition is substantially free of boron oxideand phosphorus oxide.

In another embodiment, the seal composition forms a glass ceramic sealwhich includes silica containing glass cores located in a crystallinematrix comprising barium aluminosilicate, and calcium aluminosilicatecrystals located in the glass cores.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C represent different stages of a powder composition beingconverted into a glass ceramic seal.

FIG. 2 is an SEM image of a glass ceramic seal comprising a crystallinematrix and a plurality of glass and crystalline domains within thematrix.

FIG. 3 is a cross-sectional SEM image of a seal formed on a solid oxidefuel cell component.

DETAILED DESCRIPTION

Embodiments of the invention provide a family of barium and calciumaluminosilicate (BaO—CaO—Al₂O₃—SiO₂) glass ceramic seal compositions forsolid oxide fuel cell (SOFC) stacks. The glass ceramic preferablycontains a microstructure that is stable and unchanging over theoperating life of the fuel cell, and capable of resisting prolongedexposure to air, and dry and wet fuels. The microstructure preferablyincludes a barium rich barium aluminosilicate crystalline matrixsurrounding barium lean glass regions comprising silica and calciumaluminosilicate crystallines. In other words, the microstructurecomprises a crystalline matrix surrounding glass regions or “grains”.The glass regions or “grains” may comprise discrete three dimensionalregions, such as quasi-spherical regions, which are embedded in thematrix and may not be connected to each other. The glass regions or“grains” contain additional crystallites which may comprise a differentcomposition from the crystalline matrix. This microstructure differsfrom prior art glass ceramic seal compositions which comprise a glassmatrix containing crystallites.

Furthermore, in the prior art glass and glass ceramic compositions,boron oxide is used to lower the glass transition temperature of thecompositions. However, boron oxide remains behind as the glassy matrixin which silica crystallites are located. Specifically, boron trioxidein the seal reacts with the moisture in the fuel gas mixtures to giverise to micro-bubbles in the glass, which may coalesce to larger bubblesor huge gaps in the seal structure. The loss of boron is also associatedwith crystallization of vitreous barium-aluminum-calcium-boron-silicateglass (BACS) seals, in about 1000 hours or less. The loss of boronleaves in its wake a rigid foamy structure consisting of crystallinephases, Hexacelsian, and Cristobalite. Such a structure is not expectedto be leak tight to fuel gases, and will be mechanically very fragile.

Even in glass and glass ceramic compositions of alkaline earth silicatescontaining a low content (1.5 to 6 mol. %) boron oxide, the silicatespecies generally crystallize out during sealing, leaving behind ahighly boron-enriched residual glass phase, which forms the continuousmatrix in the seal microstructure. This glass matrix is generally morefluid than the parent glass, and becomes vulnerable to boron loss andassociated problems. For example, the boron-rich glass phase can getforced out to the adjacent surfaces of the fuel cell stack underpressure leaving behind powdered crystalline mass. The boron-richresidual gas also becomes reactive with wet fuel and gets removed asvolatile boron oxyhydrides.

Preferably, the seal composition of the embodiments of the invention issubstantially free of boron or boron oxide, such as boron trioxide. Morepreferably, the seal composition is also free of phosphorus oxide andhydrogen reducible species, such as arsenic, lead or zinc oxides.

The glass ceramic seals can be formed on surfaces of fuel cell systemcomponents, such as metal or ceramic interconnect surfaces or fuel cellsurfaces, such as ceramic electrolytes of solid oxide fuel cells. Forexample, the seals may comprise ring shaped (i.e., “donut” type) sealswhich are formed on an air side of a planar metal alloy interconnect,such as a Cr—Fe alloy interconnect, around an fuel riser opening in theinterconnect. This seal prevents the fuel from flowing between the airside of the interconnect and a cathode (i.e., air) electrode of anadjacent solid oxide fuel cell. Non-limiting examples of internallymanifolded planar solid oxide fuel stacks including interconnects andsolid oxide fuel cells with fuel inlet and outlet openings which aresealed by the ring shaped seals as illustrated in U.S. application Ser.No. 12/010,884 filed on Jan. 30, 2008 which is incorporated by referenceherein in its entirety. Other interconnects and seal compositions mayalso be used.

Seal Composition

In one embodiment, a seal composition comprises (a) a first alkalineearth metal oxide, (b) a second alkaline earth metal oxide, (c) aluminumoxide and (d) silica.

The first and second alkaline earth metal oxides are different from oneanother, but otherwise can be independently chosen from oxides ofcalcium, strontium, barium and lanthanum. In a non-limiting example, thefirst and second alkaline earth metal oxides can be calcium oxide andbarium oxide, respectively. Preferably, the second oxide, such as bariumoxide, precipitates into the grain boundaries between glass particlesduring partial crystallization of the glass ceramic composition. Thus,the first alkaline earth metal oxide may comprise 100 molar percentcalcium oxide or it may comprise 80 to 99 molar percent calcium oxidesubstituted by 1 to 20 molar percent of one or both of strontium orlanthanum oxides. The second alkaline earth metal oxide may comprise 100molar percent barium oxide or it may comprise 80 to 99 molar percentbarium oxide substituted by 1 to 20 molar percent of one or both ofstrontium or lanthanum oxides.

The amount of the first and second alkaline earth metal oxides in thecomposition can vary depending on the desired properties of theresulting seal. In one embodiment, the first alkaline earth metal oxidecontent in the composition is at least about 5 mol. %. For example, thefirst alkaline earth metal oxide content can be between about 5 mol. %and about 25 mol. %, such as between about 7 mol. % and about 20 mol. %.Similarly, in one embodiment, the second alkaline earth metal oxidecontent in the composition is at least about 2 mol. %. For example, thesecond alkaline earth metal oxide content can be between about 2 mol. %and about 25 mol. %, such as between about 4 mol. % and about 20 mol. %.It should be noted that oxide content described above includes first andsecond alkaline earth metal oxides which are incorporated into binaryand/or ternary oxides in the final seal composition. If the startingmaterials for the seal composition comprise oxides, then the abovecontents may also refer to the respective starting material oxidecontents in addition to the contents in the final seal composition. Ifthe starting materials comprise carbonate or other compounds, then theabove contents may refer to final seal composition.

The relative mole ratio between alkaline earth metal oxides mayinfluence the physical properties (e.g. coefficient of thermalexpansion, “CTE”) of the resulting seal. For solid oxide fuel cellstacks containing ceramic solid oxide fuel cells having a yttria orscandia stabilized zirconia electrolytes and a Cr—Fe alloyinterconnects, the ratio of the seal oxides is selected such that thecoefficient of thermal expansion of the seal composition is betweenabout 5×10⁻⁶/° C. and about 11×10⁻⁶/° C. to match the coefficient ofthermal expansion of the fuel cells and interconnects. Accordingly, inone embodiment the relative molar ratio of the first to second alkalineearth metal oxides is at least about 1:1, respectively. In anotherembodiment, the relative mole ratio of the first to second alkalineearth metal oxides is between about 5:1 and about 1:5, respectively.

The aluminum oxide content may vary depending on the amount of the othercomponents in the composition such as the amount of silica and the firstand second alkaline earth metal oxides. For example, the aluminum oxidecontent may be provided to ensure that the ternary silicates formed fromthe composition comprise aluminum. Alternatively, the aluminum oxidecontent may be determined independently of the amounts of othercomponents. In one embodiment, the aluminum oxide content is at leastabout 5 mol. %. For example, the aluminum oxide content can be betweenabout 5 mol. % and about 15 mol. %, such as between about 7% and about13.5 mol. %. If the starting material for the seal composition comprisesaluminum oxide, then the above content may also refer to the respectivestarting material oxide content in addition to the content in the finalseal composition.

Silica is an important component of the seal composition. In the presentembodiments, the silica content in the composition is in an amount suchthat after the seal is formed from the composition, an excess amount ofsilica (also referred to as “free silica”) remains in the seal.Specifically, free silica may be considered as a silica contentremaining after the amount of silica contained in crystallized binary orternary silicates of the alkaline earth oxides is subtracted out of theinitial silica content of the glass. Without wishing to be bound to anyparticular theory, it is believed that the presence of “free” or excesssilica in the seal allows formation of a residual glass phase which aidsin holding the seal microstructure together, even though the glassceramic lacks traditional glass formers, such as boron or phosphorusoxides.

The amount of silica to be used in the composition may be determinedbased on the amount of the alkaline earth metal oxides and/or othercomponents in the composition, or determined independently. In oneembodiment, the silica content (in molar percent) is greater by at leastfive molar percent than two times the combined content (in molarpercent) of the first and second alkaline earth metal oxides. As anon-limiting example, formula (I) can be used to determine the amount ofsilica to be used in the composition:xSiO₂−2*(yA+zB)≧5 mol. %  (1)where,x=silica mol. %,y=first alkaline earth metal oxide mol. %,z=second alkaline earth metal oxide mol. %,A=first alkaline earth metal oxide (such as calcium oxide), andB=second alkaline earth metal oxide (such as barium oxide).The amount of free silica may comprise at least 5 molar percent, such as5 to 25 molar percent, for example 9 to 18 molar percent. In otherwords, xSiO₂−2*(yA+zB)≧5, such as 5 to 25 mol. %, for example 9 to 18mol. %.

In another embodiment, the silica content in the seal composition issuch that after the seal is formed, the excess or free silica content(i.e., silica that is not incorporated into a binary or ternary metaloxide) in the seal is at least about 2 mol. % of the entire seal. Forexample, the silica content in the composition can be such that afterthe seal is formed from the composition, the seal contains between about2 mol. % and about 20 mol. % free silica, such as between about 5 mol. %and about 15 mol. % free silica. In yet another embodiment, the totalsilica content (i.e., free silica and silica contained in binary andternary oxides) in the seal composition is at least about 45 mol. %. Forexample, the silica content in the composition can be between about 45mol. % and about 65 mol. %, such as between about 50 mol. % and 60 mol.%. If the starting material for the seal composition comprises silica(rather than another silicon and oxygen containing compound), then theabove content may also refer to the respective starting material oxidecontent in addition to the content in the final seal composition.

In some embodiments, the seal composition may optionally comprise anamount of a fluxing agent. A non-limiting example of a fluxing agent ispotassium oxide or strontium oxide. In general, the fluxing agent shouldnot be highly mobile as this may reduce electrical resistivity of theseal. The amount of fluxing agent may vary depending on the desiredmicrostructure of the formed seal. For example, the fluxing agentcontent in the composition can be between about 0 mol. % and about 5mol. %, such as between about 1 mol. % and about 2 mol. %. The fluxingagent may be omitted if desired.

In some embodiments, the seal composition may optionally comprise a bulknucleation additive. Examples of nucleating additives include, but arenot limited to, transition metal oxides such as TiO₂ and ZrO₂. Theamount of the nucleating additive may be between about 0 mol. % andabout 5 mol. %, such as between about 1 mol. % and 2 mol. %. Theadditive may be omitted if desired.

The seal compositions of the present embodiments can be substantiallyfree of boron oxides and phosphorus oxides. For example, substantiallyfree means that the composition may comprise less than about 0.5 mol. %boron oxide(s) and/or less than 0.5 mol. % of phosphorus oxide(s),including no boron or phosphorus oxide. Limiting the boron oxide andphosphorus oxide content in the composition is beneficial for severalreasons. As noted above, boron oxide in relatively high amounts canreact with moisture in the fuel gases to give rise to micro-bubbles inthe seal which may coalesce into larger bubbles or large gaps therebyweakening the seal structure. As another example, seals with high boronoxide content can form a boron-rich glass phase which can get forced outof the seal leaving behind a powdered crystalline mass. Moreover, aboron-rich residual gas formed by reaction of boron oxide with the fuelcan react with the fuel cell components.

Additionally, the seal compositions of the present embodiments can besubstantially free of hydrogen reducible species. Examples of hydrogenreducible species include, but are not limited to arsenic oxide, leadoxide and zinc oxide. In some cases, the composition may comprise lessthan about 0.5 mol. % of hydrogen reducible species, including zero mol.%. Again, limiting such reactive species is beneficial for maintainingthe integrity of the resulting seal.

Thus, the following non-limiting observations are made regarding thecomposition. The starting glasses for obtaining the seal structures aresilicate glasses in the true sense, because all other glass formers suchas B₂O₃, PO₅, As₂O₃, and PbO, are preferably excluded in the glassformulations. The composition also preferably excludes easily reduciblespecies such as As₂O₃, PbO and ZnO.

As noted above, the silica content of the glass, which largelydetermines the softening point of the glass, may be in the range of 50mol. % to 65 mol. %. These glasses will have their softening points inthe desired range of 750-900 C., for fuel cells operating in thetemperature range of 750 C.-900 C. Once the seal is set, thecrystallization of the glass tends to increase its softening pointsignificantly.

The high silica content of these seal compositions, and the absence ofB₂O₃ and P₂O₅ ensures that the initial glass composition does not easilycrystallize by bulk nucleation, i.e., within the glass grains. However,some high silica compositions can benefit from nucleating additives suchas TiO₂ or ZrO₂, up to 2 mol. % of these additives. The abovecomposition can also include up to 2 mol. % K₂O or other fluxing agentsfor the residual silica after the precipitation of binary and ternarysilicates of Ba and Ca. Li and Na oxides are undesirable because theirhigh mobility in glass at fuel cell operating temperatures could lead tolow electrical resistivity in the seals.

The bulk of the remaining initial glass composition should contain atleast two alkaline earth oxides, such as BaO and CaO as glass‘modifiers’. A single oxide species is not desirable because particlesof such glasses have a tendency to crystallize readily during heating,preventing the full densification of seal structure. The ratio of thetwo oxides can be varied over a wide range to manipulate CTE of theresulting seal. The larger the BaO:CaO ratio, the higher will be the CTEof the seal. Use of MgO as a modifier is not precluded but is also notdesired because it may cause premature crystallization hinderingeffective sealing.

As discussed above, the total amount of the two modifier oxides shouldnot exceed a value where the residual “free silica” is at least 5 mol.%. This amount of residual silica is desired to keep the resulting glassceramic from disintegration. Without this residual silica content, theglass particles crystallize easily during heating without firstcoalescing, to give a porous crystalline bisque which will not form ahermetic or adherent seal. Comparative example 3 below is an example ofa glass composition that is not suitable for sealing application.

Finally, the alumina in the glass is assumed to form ternary silicateswith BaO and CaO, and so is not, therefore, a factor in the calculationof ‘free-silica’. Alumina plays an important role in forming ternarysilicates with higher coefficients of thermal expansion. The aluminacontent should not exceed 15 mol. %

Seal Formation

The seal compositions described above can be used to form a seal that ishighly suited for fuel cell components. In one embodiment, a method offorming a seal comprises applying the seal compositions to one or moresurfaces and heating the composition to form a seal. Preferably thesurface is a surface of a solid oxide fuel cell component, such as aninterconnect surface. Preferably the seal is a glass ceramic material.

The seal compositions can be applied as a paste comprising glass powdersor as a green tape. Methods of making glass powders are known in theart. For example, the glass can be batched from the respective oxides orcarbonates and melted at around 1600° C., homogenized by stirring, ifnecessary, and poured out. The glass can be subsequently dry or wetmilled to obtain powdered glass. To form the paste, the powdered glasscan be mixed with suitable organic vehicles (e.g. Terpeniol), binders(e.g. Butvar) and plasticizers (e.g. benzoflex). The glass paste orslurry can also be first cast as a green tape, cut into seal shape andthen applied to a surface to form the seal.

Pastes and green tapes formed from the seal compositions can be appliedto a variety of surfaces located in a fuel cell system. Such surfacesinclude, but are not limited to, fuel cell component surfaces,interfaces between surfaces and surface cracks/imperfections. In anon-limiting example, the paste is applied to an interconnect surface.Once formed, the resulting seal can be useful, for example, to seal thefuel inlet and exhaust streams from the air inlet and exhaust streamsand from the ambient.

Methods of applying pastes or green tapes are known in the art. As anon-limiting example, the paste may be dispensed onto a surface andspread/shaped using a straight edge. In some cases, it may be desired toused both a green tape and a paste. Still, in other cases, it may bedesirable to use multiple layers of a paste and/or green tape. In suchcases, the layers may be successively formed on top of each other withor without an intermediate layer. For example, a layer of a paste orgreen tape may be applied to a surface of a fuel cell component, thenheated to form a seal, and another layer applied onto the formed seal.Alternatively, successive layers of a paste or green tape may be appliedon top of one another and the multi-layer structure can be heated toform a seal.

Heat treatments for converting the paste/green tape layer(s) into aglass ceramic seal can vary depending on the seal composition,paste/green tape thickness, desired microstructure of the seal, physicalproperties of the seal or a combination thereof. In one embodiment, thepaste or green tape is heated at a temperature sufficient to form aglass ceramic material. In another embodiment the paste or green tape isheated at a temperature sufficient to form crystalline silicates on theglass particles and/or within the glass particles. In a non-limitingexample, the seal is formed at a temperature between about 700° C. andabout 1100° C., such as between about 900° C. and about 1000° C. Theheating rate can be, for example, between about 1° C. and about 12° C.,such as between about 3° C. and about 10° C.

A conventional furnace can be used to heat the fuel cellcomponent(s)/seal composition in order to form a seal. However, it maybe possible that the operating temperature of the fuel cell (e.g. solidoxide fuel cell) stack, provides sufficient heat to form a seal from thecomposition. Additionally, in some cases, it may be desirable to place amechanical load, such as a 100 to 500 pound static load, on the sealcomposition as it is heated. For example, the seal composition may beplaced in a fuel cell stack which is compressed and heated to convertthe glass paste into a glass ceramic seal composition.

In one embodiment, the heating temperature, rate and duration allows theglass particles in the paste or green tape to first sinter (becomingglass grains), followed by formation of silicates on the glass grainboundaries, and finally formation of crystalline silicates within theglass grains. This embodiment is further illustrated in a non-limitingfashion in FIGS. 1A-C. As shown in FIG. 1A, the glass particles 2 of theseal composition initially sinter and seal by coalescence just below theglass softening point. The softening points can be, for example, betweenabout 750° C. and about 900° C. At the next stage, shown in FIG. 1B,crystallization initiating from surfaces of the glass particles forms agrain boundary network 6 of refractory silicates which are resistant toattack by fuel gases. The silicate crystalline network 6 (e.g. silicatecrystals) becoming the polycrystalline matrix surrounding the glassareas 4 (which can be referred to grains or grain cores) enclosed by thenetwork 6. In the final stage, shown in FIG. 1C, secondary crystals(e.g. silicate crystals) 8 form within the glass areas 4, resulting in aplurality of grain cores containing crystalline domains 8 within theglass areas 4 which are located in the crystalline matrix 6. Secondarysilicate crystals 8 formed in the grains may improve the mechanicalproperties and chemical resistance of the seal.

Without wishing to be bound by a particular theory, it is believed thatthe second alkaline earth oxide, such as barium oxide, precipitates intothe crystalline matrix 6 first, such that the matrix 6 is relativelybarium oxide rich compared to the areas 4 and 8. Barium oxide forms aternary oxide, such as a barium aluminosilicate crystals in the matrix6. The glass areas 4 are relatively barium oxide poor compared to matrix6 and contain free silica and calcium aluminosilicate crystals 8. Ofcourse barium oxide may be present in the grain core areas 4, 8 andcalcium oxide may be present in the matrix 6.

Without wishing to be bound by a particular theory, the crystallizationof these glasses occurs after nucleation on the surface of the glassparticles, where the free-energy for nucleation will be lower than fornucleation in the bulk. This nucleation occurs in the temperate rangebetween from glass transition temperature, T_(g) to softening point,T_(s), during heating up to the sealing temperature. This forms thepreferred seal microstructures of the embodiments of the inventioncompared to microstructures characterized by a fine crystallite networkof alkaline-earth silicates on the scale of prior glass particles, withsilica-rich residual glass remaining only in pockets or grain coreswithin the network.

A non-limiting example of a seal formed according to an embodiment isshown in FIG. 2 where the crystalline matrix or grain boundary 6comprises barium aluminosilicate, and the grain cores comprise calciumaluminosilicate crystals 8 in silica glass 4.

The seal microstructure, comprising a crystalline matrix on the scale ofthe original glass particles, enclosing pockets or grain cores ofresidual silica glass, imparts to the seal great strength, andresistance to wet fuels during fuel cell operation. The glass ceramiccompositions are tailored to set and operate at fuel cell operatingtemperatures. Seals from the powdered glass flow and wet the sealsurfaces, and crystallize during sealing producing the uniquemicrostructure described above, and stays unchanged for long timesthereafter. The viscosity of the resulting glass ceramic should be 10⁸Pa-s or above, at the stack operating temperature.

FIG. 3 is a non-limiting example of a seal formed on a fuel cellcomponent, in accordance with one embodiment. As shown, a glass ceramicseal 24 is formed on a chromium-iron alloy interconnect (fuel cellcomponent) 20 with an intermediate conductive lanthanum strontiummanganite layer 22 between the seal 24 and the interconnect 20. Theregion above the seal 26 is ambient air.

Table 1 below illustrates exemplary seal compositions according to theembodiments and comparative embodiments.

TABLE 1 Example Glass 1 2 3 4 Constituent wt. % Mol. % wt % Mol. % wt. %Mol. % wt % Mol. % CaO 15.6 19.5 15.5 19 9 12.9 8.0 11.0 BaO 16.4 7.5 94 30 15.7 27.0 13.5 Al2O3 13.8 9.5 14.5 10 17 13.3 14.0 10.5 SiO2 54.363.5 56 64 43 57.3 50.0 64.0 K2O 0 0 5 3 1 0.9 1.0 1.0 Total 100.1 100100 100 100 100.0 100.0 100.0 Free SiO2 = Mol. 9.45 18 0 15.0 % Silica −2X (Mol. % CaO + BaO)The seal compositions in table 1 are explained in more detail below.

Example 1

A seal composition has 19.5 mol. % CaO, 7.5 mol. % BaO, 9.5 mol. % Al₂O₃and 63.5 mol. % SiO₂. The calculated free silica in this composition isabout 9.45 mol. %. This composition is suitable for sealingapplications.

Example 2

A seal composition has 19.0 mol. % CaO, 4.0 mol. % BaO, 10.0 mol. %Al₂O₃, 64.0 mol. % SiO₂ and 3.0 mol. % K₂O. The calculated free silicain this composition is about 18.0 mol. %. This composition is suitablefor sealing applications.

Comparative Example 3

A seal composition has 12.9 mol. % CaO, 15.7 mol. % BaO, 13.3 mol. %Al₂O₃, 57.3 mol. % SiO₂ and 0.9 mol. % K₂O. The calculated free silicain this composition is about 0 mol. %. This composition is not ideal forsealing applications.

Example 4

A seal composition has 11.0 mol. % CaO, 13.5 mol. % BaO, 10.5 mol. %Al₂O₃, 64.0 mol. % SiO₂ and 1.0 mol. % K₂O. The calculated free silicain this composition is about 15.0 mol. %. This composition is suitablefor sealing applications.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the present invention is not so limited. It willoccur to those of ordinary skill in the art that various modificationsmay be made to the disclosed embodiments and that such modifications areintended to be within the scope of the present invention.

What is claimed is:
 1. A glass ceramic seal comprising: silica containing glass cores located in a crystalline matrix comprising barium aluminosilicate; and calcium aluminosilicate crystals located in the glass cores; wherein the seal contains zero to about 0.5 molar percent of boron oxide and phosphorus oxide and wherein the seal is formed from a composition comprising barium oxide, calcium oxide, alumina and silica in an amount such that a molar percent of silica in the composition is at least 5 molar percent greater than two times a combined molar percent of the barium oxide and the calcium oxide, and the seal is located in a fuel cell stack between a surface of an interconnect and an electrolyte of a solid oxide fuel cell.
 2. The seal of claim 1, wherein a thermal expansion coefficient of the seal is between about 5×10⁻⁶/° C. and about 11×10⁻⁶/° C.
 3. The seal of claim 1, wherein the seal comprises: between about 5 mole percent and about 20 mole percent of calcium oxide, at least a portion of which is located in the calcium aluminosilicate crystals; between about 2 mole percent and about 20 mole percent of barium oxide, at least a portion of which is located in the barium aluminosilicate crystalline matrix; between about 5 mole percent and about 15 mole percent of aluminum oxide, at least a first portion of which is located in the barium aluminosilicate crystalline matrix and at least a second portion of which is located in the calcium aluminosilicate crystals; and between about 45 mole percent and about 65 mole percent of the silica, at least a first portion of which is located in the barium aluminosilicate crystalline matrix, at least a second portion of which is located in the calcium aluminosilicate crystals, and at least a third portion of which comprises the glass cores.
 4. The seal of claim 3, wherein the composition consists of barium oxide, calcium oxide, alumina and silica and the composition contains zero to about 0.5 molar percent each of other glass formers and easily reducible species.
 5. The seal of claim 3, wherein the seal comprises zero to about 0.5 molar percent of arsenic oxide, lead oxide or zinc oxide and the seal comprises zero to about 5 molar percent at least one of a fluxing agent or a bulk nucleation agent.
 6. The seal of claim 1, wherein the seal excludes boron oxide, lead oxide and zinc oxide.
 7. The seal of claim 1, wherein the seal is made by heating a green tape, glass paste or glass slurry at a temperature between 900 and 1000° C. at a rate of between 3 and 10° C. per minute to form the glass ceramic seal. 